.. _bltin-exceptions:
Built-in Exceptions
===================
.. index::
statement: try
statement: except
In Python, all exceptions must be instances of a class that derives from
:class:`BaseException`. In a :keyword:`try` statement with an :keyword:`except`
clause that mentions a particular class, that clause also handles any exception
classes derived from that class (but not exception classes from which *it* is
derived). Two exception classes that are not related via subclassing are never
equivalent, even if they have the same name.
.. index:: statement: raise
The built-in exceptions listed below can be generated by the interpreter or
built-in functions. Except where mentioned, they have an "associated value"
indicating the detailed cause of the error. This may be a string or a tuple of
several items of information (e.g., an error code and a string explaining the
code). The associated value is usually passed as arguments to the exception
class's constructor.
User code can raise built-in exceptions. This can be used to test an exception
handler or to report an error condition "just like" the situation in which the
interpreter raises the same exception; but beware that there is nothing to
prevent user code from raising an inappropriate error.
The built-in exception classes can be sub-classed to define new exceptions;
programmers are encouraged to at least derive new exceptions from the
:exc:`Exception` class and not :exc:`BaseException`. More information on
defining exceptions is available in the Python Tutorial under
:ref:`tut-userexceptions`.
The following exceptions are used mostly as base classes for other exceptions.
.. exception:: BaseException
The base class for all built-in exceptions. It is not meant to be directly
inherited by user-defined classes (for that, use :exc:`Exception`). If
:func:`bytes` or :func:`str` is called on an instance of this class, the
representation of the argument(s) to the instance are returned, or the empty
string when there were no arguments.
.. attribute:: args
The tuple of arguments given to the exception constructor. Some built-in
exceptions (like :exc:`IOError`) expect a certain number of arguments and
assign a special meaning to the elements of this tuple, while others are
usually called only with a single string giving an error message.
.. method:: with_traceback(tb)
This method sets *tb* as the new traceback for the exception and returns
the exception object. It is usually used in exception handling code like
this::
try:
...
except SomeException:
tb = sys.exc_info()[2]
raise OtherException(...).with_traceback(tb)
.. exception:: Exception
All built-in, non-system-exiting exceptions are derived from this class. All
user-defined exceptions should also be derived from this class.
.. exception:: ArithmeticError
The base class for those built-in exceptions that are raised for various
arithmetic errors: :exc:`OverflowError`, :exc:`ZeroDivisionError`,
:exc:`FloatingPointError`.
.. exception:: BufferError
Raised when a :ref:`buffer ` related operation cannot be
performed.
.. exception:: LookupError
The base class for the exceptions that are raised when a key or index used on
a mapping or sequence is invalid: :exc:`IndexError`, :exc:`KeyError`. This
can be raised directly by :func:`codecs.lookup`.
.. exception:: EnvironmentError
The base class for exceptions that can occur outside the Python system:
:exc:`IOError`, :exc:`OSError`. When exceptions of this type are created with a
2-tuple, the first item is available on the instance's :attr:`errno` attribute
(it is assumed to be an error number), and the second item is available on the
:attr:`strerror` attribute (it is usually the associated error message). The
tuple itself is also available on the :attr:`args` attribute.
When an :exc:`EnvironmentError` exception is instantiated with a 3-tuple, the
first two items are available as above, while the third item is available on the
:attr:`filename` attribute. However, for backwards compatibility, the
:attr:`args` attribute contains only a 2-tuple of the first two constructor
arguments.
The :attr:`filename` attribute is ``None`` when this exception is created with
other than 3 arguments. The :attr:`errno` and :attr:`strerror` attributes are
also ``None`` when the instance was created with other than 2 or 3 arguments.
In this last case, :attr:`args` contains the verbatim constructor arguments as a
tuple.
The following exceptions are the exceptions that are usually raised.
.. exception:: AssertionError
.. index:: statement: assert
Raised when an :keyword:`assert` statement fails.
.. exception:: AttributeError
Raised when an attribute reference (see :ref:`attribute-references`) or
assignment fails. (When an object does not support attribute references or
attribute assignments at all, :exc:`TypeError` is raised.)
.. exception:: EOFError
Raised when one of the built-in functions (:func:`input` or :func:`raw_input`)
hits an end-of-file condition (EOF) without reading any data. (N.B.: the
:meth:`file.read` and :meth:`file.readline` methods return an empty string
when they hit EOF.)
.. exception:: FloatingPointError
Raised when a floating point operation fails. This exception is always defined,
but can only be raised when Python is configured with the
``--with-fpectl`` option, or the :const:`WANT_SIGFPE_HANDLER` symbol is
defined in the :file:`pyconfig.h` file.
.. exception:: GeneratorExit
Raise when a :term:`generator`\'s :meth:`close` method is called. It
directly inherits from :exc:`BaseException` instead of :exc:`Exception` since
it is technically not an error.
.. exception:: IOError
Raised when an I/O operation (such as the built-in :func:`print` or
:func:`open` functions or a method of a :term:`file object`) fails for an
I/O-related reason, e.g., "file not found" or "disk full".
This class is derived from :exc:`EnvironmentError`. See the discussion above
for more information on exception instance attributes.
.. exception:: ImportError
Raised when an :keyword:`import` statement fails to find the module definition
or when a ``from ... import`` fails to find a name that is to be imported.
.. exception:: IndexError
Raised when a sequence subscript is out of range. (Slice indices are
silently truncated to fall in the allowed range; if an index is not an
integer, :exc:`TypeError` is raised.)
.. XXX xref to sequences
.. exception:: KeyError
Raised when a mapping (dictionary) key is not found in the set of existing keys.
.. XXX xref to mapping objects?
.. exception:: KeyboardInterrupt
Raised when the user hits the interrupt key (normally :kbd:`Control-C` or
:kbd:`Delete`). During execution, a check for interrupts is made
regularly. The exception inherits from :exc:`BaseException` so as to not be
accidentally caught by code that catches :exc:`Exception` and thus prevent
the interpreter from exiting.
.. exception:: MemoryError
Raised when an operation runs out of memory but the situation may still be
rescued (by deleting some objects). The associated value is a string indicating
what kind of (internal) operation ran out of memory. Note that because of the
underlying memory management architecture (C's :cfunc:`malloc` function), the
interpreter may not always be able to completely recover from this situation; it
nevertheless raises an exception so that a stack traceback can be printed, in
case a run-away program was the cause.
.. exception:: NameError
Raised when a local or global name is not found. This applies only to
unqualified names. The associated value is an error message that includes the
name that could not be found.
.. exception:: NotImplementedError
This exception is derived from :exc:`RuntimeError`. In user defined base
classes, abstract methods should raise this exception when they require derived
classes to override the method.
.. exception:: OSError
.. index:: module: errno
This exception is derived from :exc:`EnvironmentError`. It is raised when a
function returns a system-related error (not for illegal argument types or
other incidental errors). The :attr:`errno` attribute is a numeric error
code from :cdata:`errno`, and the :attr:`strerror` attribute is the
corresponding string, as would be printed by the C function :cfunc:`perror`.
See the module :mod:`errno`, which contains names for the error codes defined
by the underlying operating system.
For exceptions that involve a file system path (such as :func:`chdir` or
:func:`unlink`), the exception instance will contain a third attribute,
:attr:`filename`, which is the file name passed to the function.
.. exception:: OverflowError
Raised when the result of an arithmetic operation is too large to be
represented. This cannot occur for integers (which would rather raise
:exc:`MemoryError` than give up). Because of the lack of standardization of
floating point exception handling in C, most floating point operations also
aren't checked.
.. exception:: ReferenceError
This exception is raised when a weak reference proxy, created by the
:func:`weakref.proxy` function, is used to access an attribute of the referent
after it has been garbage collected. For more information on weak references,
see the :mod:`weakref` module.
.. exception:: RuntimeError
Raised when an error is detected that doesn't fall in any of the other
categories. The associated value is a string indicating what precisely went
wrong. (This exception is mostly a relic from a previous version of the
interpreter; it is not used very much any more.)
.. exception:: StopIteration
Raised by builtin :func:`next` and an :term:`iterator`\'s :meth:`__next__`
method to signal that there are no further values.
.. exception:: SyntaxError
Raised when the parser encounters a syntax error. This may occur in an
:keyword:`import` statement, in a call to the built-in functions :func:`exec`
or :func:`eval`, or when reading the initial script or standard input
(also interactively).
Instances of this class have attributes :attr:`filename`, :attr:`lineno`,
:attr:`offset` and :attr:`text` for easier access to the details. :func:`str`
of the exception instance returns only the message.
.. exception:: IndentationError
Base class for syntax errors related to incorrect indentation. This is a
subclass of :exc:`SyntaxError`.
.. exception:: TabError
Raised when indentation contains an inconsistent use of tabs and spaces.
This is a subclass of :exc:`IndentationError`.
.. exception:: SystemError
Raised when the interpreter finds an internal error, but the situation does not
look so serious to cause it to abandon all hope. The associated value is a
string indicating what went wrong (in low-level terms).
You should report this to the author or maintainer of your Python interpreter.
Be sure to report the version of the Python interpreter (``sys.version``; it is
also printed at the start of an interactive Python session), the exact error
message (the exception's associated value) and if possible the source of the
program that triggered the error.
.. exception:: SystemExit
This exception is raised by the :func:`sys.exit` function. When it is not
handled, the Python interpreter exits; no stack traceback is printed. If the
associated value is an integer, it specifies the system exit status (passed
to C's :cfunc:`exit` function); if it is ``None``, the exit status is zero;
if it has another type (such as a string), the object's value is printed and
the exit status is one.
Instances have an attribute :attr:`code` which is set to the proposed exit
status or error message (defaulting to ``None``). Also, this exception derives
directly from :exc:`BaseException` and not :exc:`Exception`, since it is not
technically an error.
A call to :func:`sys.exit` is translated into an exception so that clean-up
handlers (:keyword:`finally` clauses of :keyword:`try` statements) can be
executed, and so that a debugger can execute a script without running the risk
of losing control. The :func:`os._exit` function can be used if it is
absolutely positively necessary to exit immediately (for example, in the child
process after a call to :func:`fork`).
The exception inherits from :exc:`BaseException` instead of :exc:`Exception` so
that it is not accidentally caught by code that catches :exc:`Exception`. This
allows the exception to properly propagate up and cause the interpreter to exit.
.. exception:: TypeError
Raised when an operation or function is applied to an object of inappropriate
type. The associated value is a string giving details about the type mismatch.
.. exception:: UnboundLocalError
Raised when a reference is made to a local variable in a function or method, but
no value has been bound to that variable. This is a subclass of
:exc:`NameError`.
.. exception:: UnicodeError
Raised when a Unicode-related encoding or decoding error occurs. It is a
subclass of :exc:`ValueError`.
.. exception:: UnicodeEncodeError
Raised when a Unicode-related error occurs during encoding. It is a subclass of
:exc:`UnicodeError`.
.. exception:: UnicodeDecodeError
Raised when a Unicode-related error occurs during decoding. It is a subclass of
:exc:`UnicodeError`.
.. exception:: UnicodeTranslateError
Raised when a Unicode-related error occurs during translating. It is a subclass
of :exc:`UnicodeError`.
.. exception:: ValueError
Raised when a built-in operation or function receives an argument that has the
right type but an inappropriate value, and the situation is not described by a
more precise exception such as :exc:`IndexError`.
.. exception:: VMSError
Only available on VMS. Raised when a VMS-specific error occurs.
.. exception:: WindowsError
Raised when a Windows-specific error occurs or when the error number does not
correspond to an :cdata:`errno` value. The :attr:`winerror` and
:attr:`strerror` values are created from the return values of the
:cfunc:`GetLastError` and :cfunc:`FormatMessage` functions from the Windows
Platform API. The :attr:`errno` value maps the :attr:`winerror` value to
corresponding ``errno.h`` values. This is a subclass of :exc:`OSError`.
.. exception:: ZeroDivisionError
Raised when the second argument of a division or modulo operation is zero. The
associated value is a string indicating the type of the operands and the
operation.
The following exceptions are used as warning categories; see the :mod:`warnings`
module for more information.
.. exception:: Warning
Base class for warning categories.
.. exception:: UserWarning
Base class for warnings generated by user code.
.. exception:: DeprecationWarning
Base class for warnings about deprecated features.
.. exception:: PendingDeprecationWarning
Base class for warnings about features which will be deprecated in the future.
.. exception:: SyntaxWarning
Base class for warnings about dubious syntax
.. exception:: RuntimeWarning
Base class for warnings about dubious runtime behavior.
.. exception:: FutureWarning
Base class for warnings about constructs that will change semantically in the
future.
.. exception:: ImportWarning
Base class for warnings about probable mistakes in module imports.
.. exception:: UnicodeWarning
Base class for warnings related to Unicode.
.. exception:: BytesWarning
Base class for warnings related to :class:`bytes` and :class:`buffer`.
Exception hierarchy
-------------------
The class hierarchy for built-in exceptions is:
.. literalinclude:: ../../Lib/test/exception_hierarchy.txt